How to support class methods in interface definitions in Delphi

Question

I created a class method in a class that implements an interface, but I can't seem to define it inside of the interface.

IMyClass = interface
  procedure someproc();
  class function myfunc() : TMyClass;  // compiler doesn't like this!
end;

TMyClass = class( TInterfacedObject, IMyClass )
public
  procedure someproc();
  class function myfunc() : TMyClass;
end;

I want myfunc() to create and return an instance of TMyClass. For example:

somefunc( TMyClass.myfunc(), ... );

creates an instance of TMyClass and passes it into somefunc.

I can define function myfunc() : TMyClass; in the IMyClass interface, but if I put class in front of it, the compiler gives me an error. If I leave it off, it gives me several other errors "E2291 Missing implementation of interface method xyz.myfunc" It just doesn't accept the same signature in the interface as in the class.

I thought I've seen this work before (class methods defined in interfaces) but maybe not.

If this isn't supported directly, how do you do it?

(I'm using Delphi XE5, in case it matters.)

Answer 1

Interfaces are not classes and do not support methods marked as class, they only support non-class methods that are implemented and called on object instances.

What you are looking for will most likely have to be implemented as a class factory instead of using an interface.